Like most stuff it's a compromise.
It's easy to do initial roughing out as a rectangular cross section. It's also easier to remove small amounts in a controlled manner from a corner rather than a flat surface.
Bearing in mind the final bow will have rounded corners, it's a matter of when you start rounding.
I tend to get a bow pulling back say 20" on along string before I start to round the corners.
It amuses me slightly that some people get obsessed with the "rounded belly" of an English Longbow. It's not necessarily very rounded and many of the Mary Rose bows are more like a slightly inflated rectangle or square in cross section. Simply taking off the corners produces the shape.
I find it's a bit like reducing the thickness of a log using an axe, it's easier to cut off the corners first, and then the resulting ridge that runs down the middle. (see pic)
Go with your gut... there's no right or wrong.
If it's final finishing that you are worried about, just round 'em off a tad til they look and feel right, a little rounding won't effect the draw weight and will minimise the risk of splinters. A slight curve on the belly is usual, but some like 'em flat or even concave.